Conventional aluminium cold rolling mills typically use kerosene as a coolant. This contains a small amount of lubricant also. The kerosene is sprayed onto the rollers using a spray bar including a number of nozzles. Thousands of liters are used to cool the rollers, which heat up due to work input into the aluminium by the rollers. The kerosene is recirculated through a filter system and is cooled to about 40 degrees Celsius. It nonetheless poses a significant fire risk. Fires may be extinguished by CO2 systems, but these need to be large and are expensive.
Water is an attractive coolant because it poses no fire risk and has good specific heat properties. However, water left in contact with aluminium damages the “mirror” finish of the aluminium, causing local corrosion, particularly if it gets trapped in the rolled foil.
An alternative coolant is liquid nitrogen (LN2). This cannot be recycled. But, on a large scale it is sufficiently inexpensive. LN2 has an advantage in that it separates the cooling medium from the lubrication medium. In comparison, kerosene with lubricant included cannot achieve this. When rolling thin films (e.g. 0.1 millimeters or less), the viscosity of the lubricant has a major impact on the speed of rolling that is possible. This is because the thickness of a lubrication film between the rolls and the strip being rolled is determined by a hydrodynamic effect. The rollers contact each other outboard of the strip width and the foil actually deforms the rollers in use. The actual foil thickness is controlled by the speed of rolling and the lubricant viscosity (remembering the rolls actually contact each other in the absence of the foil). This effect is highly significant in thin foils. So for thin foils, it is preferable to use low viscosity lubricant. For thicker material, high viscosity is better because this helps to maximize the “reduction” through the mill bite. Kerosene does not allow this control because the lubricant is incorporated into the coolant.
LN2 cooling tends to cause water to condense out of the air. Hence, a shroud is needed. An example of an arrangement including a shroud is disclosed in WO-2012/110241. Inside the shroud only nitrogen is present. However, it is also necessary to warm the shroud (for example, electrically or using a gas within the shroud) to ensure there is no condensation on the outside of the shroud which could get into the mill. A difficulty with the use of such a system is how to “seal” the shroud against the rotating roll. It is not possible to have physical contact between them because any contact (e.g. rubber) would damage the mirror surface of the foil. So a gas curtain or an air knife type effect is used. It has been found that a gap between the shroud and the roll has to be about 1 to 2 millimeters to ensure an effective seal with acceptable gas consumption. The roll length is about two meters, and the shroud is only supported at each end of the roll and so it is difficult to achieve accurate tolerances for this gap across the full length which can upset the effectiveness of the gas curtain.
Rollers need to be changed quite often. This involves the rollers generally being retracted axially out of the mill as a pair. The rollers are mounted in “chocks” and the whole chock system with rollers is removed. A problem is that a shroud, which, due to tolerances, must be mounted to the chocks, is too big to be retracted from the mill along with the rollers. Moreover, there is not much room to maneuver in the vicinity of the shroud because there may be thickness and flatness detectors in the way, along with “bend blocks” which are used to change the orientation of the rolls by adjusting the chock positions. Even if the shroud could be so removed, it would still be necessary for all the gas lines to be reconnected.
The present invention has a goal of alleviating at least to some extent one or more of the problems of the prior art.